The use of mobile communications networks has increased over the last decade. Operators of mobile communication networks have increased the number of base stations in order to meet an increased request for service by users of mobile communications networks. It is of interest for the operator of the mobile communications network to reduce the running costs of the base stations. It is one option to implement the radio system as an antenna embedded radio system. In the antenna embedded radio system, some of the hardware components of the radio system may be implemented on a chip. The antenna embedded radio system therefore reduces running costs of the base station. Implementing the radio system as the antenna embedded radio system reduces space needed to house the hardware components of the base station. Power consumption during normal operation of the radio system is reduced when implanting the antenna invaded radio system comprising the chip.
It is of interest to provide a reliable quality of service to individual users of the mobile communication network given the increase in the number of users. Several techniques have been suggested in order to deal with the increased number of users within the mobile communications network. One of these several techniques is the provision of beam forming capabilities in order to direct a beam relayed by the radio system in different directions to improve service coverage of the mobile communications network. The beam forming techniques rely on defined phase and amplitude relations between several ones of antenna elements of the active antenna system. Delays along a transmit path and/or a receive path (commonly termed “signal path”) may cause a delay for signals travelling along the signal path. Likewise phase and amplitude of the signal travelling along the signal path may change when the signal travels along the signal path. For some applications it is of interest to calibrate a transmit time or the delay accumulated along the signal path. The calibration of the delay is of interest, should the delay vary with conditions of the signal path, such as but not limited to start-up conditions of the signal path. A change in the delay depending on the start-up conditions is, for example, relevant for the signal paths relaying digital signals. Buffers in the digital signal path may introduce the variable delay depending on the start-up conditions. Frequently the signal path may be operating at a higher frequency and a higher sampling rate than the signal entering the signal path.
The prior art discloses the examples of the measurement of the delay introduced by the signal path at a coarse sampling rate corresponding to the lower frequency of the signal entering the signal path.
Several concepts for calibrating an active antenna system comprising several signal paths relaying signals in a well-defined fashion are known. For example, “Investigation of a calibration concept of optimum performance of adapted antenna systems” by Passmann et al. of Robert Bosch GmbH at Vehicular Technology Conference, 1998, VTC98, p. 577-580, discloses a calibration concept for an adaptive antenna system. The calibration concept consists of (i) offline calibration of passive components after the manufacture; and (ii) an on-line calibration of active components during operation.
Korean Patent Application Number 1020050089853A (assigned to DA TANG Mobile Communications) teaches a method for calibrating smart antenna array systems in real time. The DA TANG application teaches a iterative method of calibrating smart antenna array systems using compensation factors of previous calibrations in order to calibrate transmit and receive paths.
U.S. Pat. No. 6,693,588 B1 (assigned to Siemens) discloses an electronically phase controlled group antenna. The electronically phase controlled group antenna is calibrated using a reference point shared by all the reference signals. In the downlink reference signals which can be distinguished from one another are simultaneously transmitted by individual antenna elements of the group antenna and suitably separated after reception at the shared reference. The Siemens system requires a fixed special arrangement of the antenna elements.
International Patent Application Number WO 2007/049023 A1 (assigned to Mitsubishi Ltd.) teaches a signal processing and time delay measurement based on combined correlation and differential correlation. Two versions of a binary signal having an irregular sequence of states are processed by dividing (i) the first value which represents the average time derivative of one signal at the times of transitions in the other signal, and (ii) a correlation value for the two signals, and then combining the first value with the correlation value. For a given relative delay introduced between the signals, the resultant combined value indicates whether the introduced delay brings the transitions in the two signals into coincidence. This process can be repeated for other introduced delays to determine the amount of delay between the two signals.
UK Patent Application Number GB 2 447 981 A (assigned to Mitsubishi electric information technology Centre Europe B. V.) discloses a time delay measurement for global navigations satellite system receivers. A method is provided for processing first and second signals having a delay there between, whereby at least the first signal is an irregular binary signal having chip boundaries. The method comprises introducing a plurality of different delays between the first and second signals, the successive delay amounts differing from each other by less than the interval between chip boundaries. For each introduced delay, summing the samples of the second signal which are obtained at the times when the chip boundaries between bits of the first signal have the same state (i.e. a transition from 1 to 0, from 1 to 0, from 0 to 0, or from 1 to 1) in order to obtain a value, and thereby obtaining a representation of how the value varies according to the introduced delay, such that the representation contains a level change associated with an introduced delay which bears a predetermined relationship to the delay between the first and the second signal. Values corresponding to different transitions may be subtracted from one another. The method is particularly suited for processing signals in GPS or other similar positioning signals, for example, in a tracking loop phase error discriminator for DLL code alignment, or in line of sight signal (LoS) signal timing recovery from multi path contamination.